1. Field of the Invention
The present invention relates to an agricultural haymaking machine intended to be hitched to the lower arms of a tractor and comprising:                a frame supported on the ground by wheels,        at least one work device connected to the frame, the frame being in pivoting connection in its front part with a hitching device configured to be connected to the two lower arms, the center of gravity of the machine being situated between the wheels and the hitching device.        
2. Discussion of the Background
Such a machine is known from the document FR 2 778 311. This document describes a windrowing machine of the semi-mounted type which comprises a frame, the front part of which is in pivoting connection with a hitching device configured to be hitched to the two lower arms situated at the rear of a tractor. The pivoting connection has a substantially vertical axis. The frame is supported in its rear part by two wheels. Two work devices are articulated on the frame, one is arranged with respect to the frame on the left-hand side in the direction of advance and the other on the right-hand side, and each comprises its own train of wheels which has been chosen to be designated as carrier wheels in the rest of the text so as to distinguish them from the wheels supporting the rear part of the frame. Each of the work devices can be moved between a work position, in which it extends laterally following a direction substantially perpendicular to the direction of advance, and a transport position in which it is raised substantially to the vertical.
The machine previously described presents a certain number of drawbacks, the exposition of which requires a recollection of the definition of the sustentation triangle. This imaginary triangle has a first side formed by a vertical plane connecting the center of the left wheel to the center of the right wheel. It has a second side which, when the machine is in a straight line, is formed by a vertical plane passing through the center of the left wheel and through the articulation between the hitching device and the lower left arm of the tractor, whilst the third side is formed by a vertical plane passing through the center of the right wheel and through the articulation between the hitching device and the lower right arm of the tractor. On a substantially flat terrain and in a straight line, the center of gravity of the machine is located inside the sustentation triangle. However, when the machine undergoes a lateral acceleration, it can occur that the machine rests on only one of the lower arms, in which case the previously described machine has a major defect. By way of example, if the machine undergoes a lateral acceleration oriented towards the left, it remains stable if it rests on the lower left arm, whereas it risks tilting if it rests on the lower right arm. Now, the fact that the above-mentioned pivoting connection has a substantially vertical axis prevents a prediction being made as to which of the lower arms the machine will rest on when it undergoes a lateral acceleration. This uncertainty represents a risk for the stability of the machine, which is aggravated by the fact that, when the machine is in the transport position, its center of gravity is situated at a great distance from the ground owing to the vertical extension of the work devices.
During transport, the machine can be brought to follow a curved course, for example on cornering or on a half-turn at the end of the plot of land. In this situation, the centrifugal force can cause the center of gravity to leave the sustentation triangle, which causes the lateral tilting of the machine. This phenomenon is all the more marked, the greater the running speed and the smaller the radius of curvature of the course. The phenomenon will be further aggravated if the maneuver is carried out on a field having numerous hollows and bumps, or again on a sloping field and where the side of the machine situated towards the exterior of the curved course is lower, due to the slope, than the opposite side. The result of this is that the farmer must slow the tractor down greatly on turns. In addition, he must operate the half-turn maneuvers at a substantially reduced speed and/or with a large turning radius, the latter generating additional maneuvers.
During work, such a machine often allows the work devices to be raised a small distance from the ground so that they do not come in contact with the fodder lying on the ground. In this raised position, the train of carrier wheels of each work device does not touch the ground. The result of this is that the only wheels which carry the machine are those at the rear of the frame. But, the work devices in the raised position extend laterally much more than the wheels at the rear of the frame. Thus, when the machine follows a curved course, it is even more easily destabilized than in the transport position. To reduce this risk, the farmer must travel at a reduced speed over his plot of land, which slows him down in his work.